1. Field of the Invention
The present invention relates to an image recording apparatus, and more particularly, it relates to a heat transfer colored image recording apparatus.
2. Related Background Art
In general, a thermal recording apparatus of heat transfer type is so designed that an ink sheet and an image receiving paper (transfer paper) are overlapped and heat or thermal pulses are applied to the ink sheet by means of a thermal head. Since the ink which can be liquidized or sublimated by the heat is coated on the ink sheet, the ink on the ink sheet where the thermal pulses are applied is transferred to the image receiving paper, thus recording image information corresponding to the thermal pulses on the image receiving paper. Because the thermal head is of the line printing type, in order to record the image or images on the whole area of the image receiving paper, it is necessary to shift the thermal head and the image receiving paper relatively.
Conventionally, in many cases, the thermal head is fixedly mounted so a not to be shifted and the image receiving paper is shifted with respect to the thermal head. The following techniques are typical of the prior art: a drum fixing technique wherein one end of the image receiving paper is fixed to a drum by means of a pawl and the like and the thermal head is pressed against the image receiving paper with the interposition of the ink sheet and the drum is rotated to shift or rotate the image receiving paper together with the ink sheet, thereby printing the image on the image receiving paper; a pinch roller technique wherein the image receiving paper is pinched between the drum and a pinch roller and the thermal head is pressed against the image receiving pape,r thereby printing the image on the image receiving paper; or a capstan roller technique wherein the image receiving paper is pinched between the thermal head and a small rubber roller and the image receiving paper is shifted or fed by means of a separate capstan rollers.
However, each of the above-mentioned techniques has both merit and demerit with respect to the size and/or cost of the recording apparatus and with respect to chromatic shear in printing, and, thus, conventionally, one of these techniques has been selected in accordance with the purpose of the image recording apparatus to be used.
Recently, a new technique called a "one-way circulating feed system" has been proposed, independently of the above-mentioned techniques, which could eliminate the demerits of the above-mentioned three techniques.
Explaining this new system with reference to FIG. 5, a thermal head 1 is pressed against a platen (main roller) 2 with the interposition of an ink sheet 4 and an image receiving paper 5. The ink sheet 4 is coated by paint. An auxiliary roller 3 serves to circulate the image receiving paper 5. The ink sheet 4 is fed from a feed roll 31 to a take up roll 30 through guide rollers 7 and 8. The image receiving paper 5 is introduced from a paper inlet 6. Pinch rollers 16-19 serve to press the image receiving sheet 5 against the platen 2 and auxiliary roller 3. The image receiving paper 5 is guided by guides 10-15. An optical sensor 9 detects a leading edge of the image receiving paper 5 to determine the timing of initiation of printing or recording.
In operation, when the image receiving paper 5 is introduced from the paper inlet 6, the platen 2 and auxiliary roller 3 start to rotate, with the result that the image receiving paper 5 is passed between the guides 11 and 12 and then is wound around the platen 2. When the leading edge of the image receiving paper 5 reaches the optical sensor 9, a signal is emitted to start the printing. The image receiving paper 5 being printed is then passed between the guides 14 and 15 and is wound around the auxiliary roller 3. Subsequently, the same image receiving paper 5 is guided by the guides 11, 12 again to reach the platen 2. When the printing or recording with a first color is completed, the leading edge of the image receiving paper 5 is positioned near the thermal head 1. Further, when the leading edge of the image receiving paper reaches the optical sensor 9 again, the printing with a second color is initiated. In this way, the printing operations with the second, third and fourth colors are repeated to finally obtain a colored image.
Although such an arrangement is meritorious in that the whole apparatus can be small-sized and the leading edge of the image receiving paper can be positioned near the thermal head immediately after each printing operation is completed, there arises a drawback in that chromatic shear in printing occurs when an image with each of the third and fourth colors is overlapped on the previously formed image. Such chromatic shear in printing occurs as follows: even when the image receiving paper is properly wound on the platen, i.e., even when the leading edge of the image receiving paper is parallel to an axis of rotation of the platen 2, if the leading edge of the image receiving paper when wound on the auxiliary roller 3 is not substantially parallel to an axis of rotation of the auxiliary roller, the image receiving paper will deviate from a paper feeding direction transversely. Such deviation is caused by the fact that the platen and/or auxiliary roller is not completely cylindrical and that the axis of rotation of the platen is not completely parallel to the axis of rotation of the auxiliary roller. As a result, shear in printing occurs due to such deviation of the image receiving paper, which leads to deterioration of image quality.